Wireless communication systems typically provide services to a subscriber coverage area based on a division of the coverage area into areas referred to as cells. Typically the cells are further divided into sectors, and portions of the available radio bandwidth are assigned to each of the sectors. A group of cells that uses all available channel frequencies is generally referred to a cluster, and the number of cells per cluster (N) is a measure of frequency reuse. Because the number of available frequencies is limited, efficient frequency reuse is an important consideration in wireless network layout. Cell layout is generally selected based on providing acceptable communication throughout the coverage area while avoiding interference between signals associated with the same frequency but produced in different cells. Each cell is typically associated with a cell site at which antennas for the cell sectors are located.
FIG. 1 illustrates a so-called wide-beam trisector division of a cellular coverage area 100. The coverage area 100 is divided into a plurality of hexagonal cells and an available radio bandwidth is divided into 12 different channel frequencies f1, . . . , f12. Three channel frequencies are assigned to each of the cells. As shown in FIG. 1, cells 102, 104, 106, 108 use frequencies f1-f3, f4-f6, f7-f9, and f10-f12, respectively, and form a first cluster 111. Thus, for the arrangement of FIG. 1, N=4. Additional cell clusters 112, 113, 114 are provided to extend the cellular coverage area. As shown in FIG. 1, the cells are associated with three channel frequencies and three corresponding 120 degree antennas. The antennas are configured to communicate with corresponding sectors of the hexagonal cell areas using different channel frequencies. For example, antennas situated in the cell 102 are arranged to have transmission/reception directions 116, 117, 118 configured to service sectors 126, 127, 128, respectively. While this arrangement reduces co-channel interference, the coverage area 100 includes so-called dead spots such as the representative dead spots 120, 122, 124. These dead spots correspond to off-axis portions of antenna radiation patterns and are associated with reduced radio signal strength in comparison with other portions of the cellular coverage area 100.
FIG. 2 illustrates a cellular coverage area 200 divided according to a so-called narrow-beam trisector configuration in which each cell includes three 60 degree directional antennas that are assigned different channel frequencies and configured to serve corresponding hexagonal coverage areas. For example, the coverage area 200 includes a representative cluster 201 that includes so-called “cloverleaf” cells 202, 204, 206, 208. The cell 202 includes sectors 210, 212, 214 that are serviced by channel frequencies communicated along axes 216, 218, 220, respectively, typically using antennas having 60 degree beamwidths. The cluster 201 includes N=4 cells and each of the cells 202, 204, 206, 208 is assigned three channel frequencies so that this configuration is associated with 4 by 12 frequency reuse.
The coverage area 200 as divided according to FIG. 2 does not exhibit the dead spots associated with the wide-beam configuration of FIG, 1, but exhibits different limitations. Referring to cells 230, 232, a selected channel frequency is communicated along antenna axes 231, 233 in sectors 236, 238, respectively. Thus, a signal radiated along the antenna axis 233 in the cell 232 is also received in the cell 230 along the antenna axis 231. Thus, reuse of the channel frequency associated with the antenna axis 233 in the cell 232 results in so-called co-channel interference in the cell 230. This co-channel interference is caused by reception of a signal intended for a recipient in the sector 238 but received in the sector 236.
As shown in FIGS. 1-2, the arrangement of cells in a wireless communication network is generally based on reuse of channel frequencies to increase network capacity and improve network performance. Unfortunately, cellular arrangements such as those of FIGS. 1-2 exhibit unacceptable dead zones or unacceptable levels of co-channel interference. Thus, increasing frequency reuse to extend network capacity produces degraded received signal quality, and existing networks often exhibit dead zones or noticeable levels of co-channel interference. Because only limited bandwidth is available for most wireless systems, co-channel interference is a significant limitation on system data rate and number of subscribers served. Thus, systems and methods that reduce co-channel interference without introducing dead zones are needed.